The following abbreviations are herewith defined:
3G Third Generation Mobile Network
AR access router
ARQ automatic repeat request
BS base station (also referred to as a Node B)
E-UTRAN Evolved Universal Terrestrial Radio Access Network
HSDPA High Speed Downlink Packet Access
IP internet protocol
L1 Layer 1 (Physical Layer)
L2 Layer 2 (MAC Layer)
LCID logical channel identity
MAC Medium Access Control Layer (L2)
PHY Physical Layer (L1)
PDU protocol data unit
QoS quality of service
RNC radio network controller
SDU service data unit
SSN service data unit (SDU) sequence number
TB transport block
TCP transmission control protocol
UDP user datagram protocol
UE user equipment
UL uplink
UTRAN Universal Terrestrial Radio Access Network
VoIP voice over internet protocol
WCDMA Wide-Band Code-Division Multiple Access
WLAN wireless local area network
In E-UTRAN, application flows with different QoS requirements are served over the wireless path by different logical channels in the MAC protocol layer. MAC SDUs, which are higher layer packets such as IP packets, are queued in priority queues, which are arranged for the logical channels. The amount of data to be transmitted for each logical channel is determined for every radio frame transmission, attempting to meet the QoS requirements of each IP traffic flow. Then, for each UE, MAC multiplexes (concatenates) the scheduled data from the priority queues into one TB. In this process, MAC may need to segment MAC SDUs to make them fit into the TB. After triggering TBs from MAC, PHY multiplexes TBs from different LJEs into a radio frame.
In prior art cellular systems (e.g., 3G), the SDUs are segmented and concatenated to constant size PDUs, which are defined per transport channel. This adds segmentation and multiplexing overhead. The reasoning is that the transmission capacity of a radio link varies in time and small payloads are often available. Thus the constant size PDU typically needs to be small. A small PDU fits well with the small rate channels, but will cause a lot of overhead when segmenting large SDUs into the small PDUs. On the other hand, many small PDUs need to be created for the high rate channels, which will cause multiplexing overhead. Optimally, the PDU size would be modified depending on the capabilities of the transport channel and its temporary conditions. However, modification of the PDU size, in 3G, requires a heavy peer-to-peer procedure and re-segmentation. Hence, it is typically not preferred.
In prior art wireless systems (e.g., WLAN), SDUs are transmitted as full packets. The multiple access is based on random access/collision detection in uplink and scheduling in downlink. Thus, once a transmission resource is indicated for a given user, it is allowed to use the full bandwidth for a short period of time as required for the transmission of the entire SDU available. In such a manner, there is less segmentation and multiplexing overhead. However, large expected multi-user multiplexing gains will not be available.
The problems of these prior art segmentation schemes are even more evident in newer cellular and wireless systems, where the available bandwidth is large, bandwidth flexibility is large and the symbol rate is high, but varying radio conditions impose receiver-dependent and time/frequency-dependent characteristics on the transmission of each radio link. On the other hand, for any receiver, gains available by frequency scheduling, gains available by exploiting the frequency diversity present in the channel and gains available by adaptive transmission bandwidth selection are significant. Yet, multi-user gains, which are realized by allocating independent radio links efficiently in time and frequency are significant as well. Thus, the segmentation scheme should be flexible and efficient to allow usage of any of these kinds of transmission techniques. Neither of the mentioned prior art schemes, i.e. a fixed PDU size and a trivial segmentation scheme, can efficiently meet these contradictory requirements. In such conditions, full SDU transmissions are feasible and generally preferred for low overhead. However, segmentation may still be necessary for large SDUs to be received on difficult low bit-rate radio links.
In conventional segmentation approaches for WCDMA and HSDPA, segmentation is performed before a TB size is decided. Hence, the system can only deliver fixed size or at least ready-made segments and thus has to concatenate segments to fill the TB efficiently. This increases the amount of headers and complicates the procedure by trying to match segments to the tail of the TB.
In another prior art segmentation scheme, SDU retransmissions using the SSN is employed. However, full SDU retransmission is generally inefficient and may lead to problems in low bit-rate radio link conditions. The efficiency also depends on the traffic class and the size distribution of data. If the application generates large TCP/UDP segments in the IP packets and the system bandwidth is narrow, one SDU must be segmented into many small segments. For example, the maximum transmission unit (MTU) or maximum segment size (MSS) for an IP packet over Ethernet is typically 1500 bytes, and one sub-frame over a 1.25 MHz system with ½-coding rate and Quadrature Phase Shift Keying (QPSK) modulation only has about 450 information bits. This means that, for this system, one SDU will be segmented into 28 segments, thus increasing the probability of SDU error. A large SDU in such a system would likely be re-transmitted one or more times. Not only will the throughput of the radio link be significantly reduced but, in addition, the cell throughput will be reduced since retransmissions are typically prioritized.